Move our DC to implement mandetory signing.

Move our DC to implement mandetory signing.

(this does not change the file server role, and only really changes
what 'server signing = auto' means)

Optional signing really isn't any benifit to network security.

In doing so, allow anonymous clients (if permitted by policy) to log
in without signing, as Samba3 does not sign these connections (which
would use an all-zero key, so pointless).

Andrew Bartlett
(This used to be commit 468bf839c500ed1a26ab9a358ee64a4c0a695797)